Optical spectrometers are systems that permit the measurement of optical intensity at specific wavelengths within a spectral region. In broad terms, a spectrometer can either measure each of the wavelengths individually, or it can measure multiple portions of the spectrum at one time. In the first case, the spectrometer operates in a sequential mode and is referred to as a scanning spectrometer system. In the latter case, the spectrometer is said to operate in a multiplexed fashion. Multiplexed spectrometers can be further divided into those that are based upon a detector array and those that use just a single detector. Array-based spectrometers incorporate some optical element such as a grating or prism to separate the light and distribute it across the detector array to record the individual intensities incident on the detector pixels. In contrast, a single-element multiplexing spectrometer incorporates an optical encoding subsystem such as an interferometer to encode the light in a manner that the resulting signal can be processed after collection to regenerate the individual spectral intensities. The subject of this disclosure is a new method for constructing a multiplexing spectrometer that is based on a single element detector, an optical filter assembly, and an encoding mask.
Spectroscopic measurement systems operating in the visible or near-infrared spectral regions may be used to measure a wide variety of sample characteristics that convey information regarding the presence and/or quantity of analytes. In particular, they can be used to measure analytes in inanimate samples, biological samples, and in human subjects. They can provide identifying information about the person or other sample type, provide information about the age, gender, or other demographic factors about the person being measured, provide information about the disease state of the person or sample being measured, or provide information about the quality or similarity of the sample being measured relative to some known population. In order to design and build a commercially viable spectroscopic system for these types of measurements, the spectrometer should be multiplexed and have high optical throughput in order to allow the spectroscopic system to collect as much light as possible in a given measurement time, and thereby increase the total measurement signal-to-noise ratio. The spectrometer system should also be stable to reduce the effect of instrument drift and reduce noise. Also, in order to facilitate the ability to perform a calibration on one instrument and use the same calibration model for other similar instruments (i.e. calibration transfer), the spectrometer system should be of a simple and robust design with a minimum number of components that have critical dimensions or require precise movement.
In addition, in order to facilitate efficient manufacture of such a spectrometer, it should contain few parts with a minimal number of adjustable critical dimensions. Ideally, the parts of the spectrometer would be well suited for large-scale manufacturing processes in order to fulfill the demands of large-volume production. It is also desirable that the parts of the spectrometer are manufactured using well-developed and well-understood technologies and materials to avoid unexpected interactions and effects when the parts are assembled into a complete spectrometer.
For broad commercial viability, the spectrometer should be designed to be as small and as inexpensive as possible. Due to the high cost of near-infrared detector arrays, a near-infrared spectrometer suitable for low-cost applications preferably will be based upon a single-element detector. In addition, the spectroscopic system must be rugged to withstand shock, dust, humidity and other adverse environmental influences.
There is a variety of spectrometers capable of being incorporated in a spectroscopic system to produce optical spectra which potentially can be used for analysis of analytes in biological media. Examples of suitable spectrometers include: diffraction spectrometers utilizing scanned or array diffraction gratings; refraction spectrometers utilizing a prism or mock interferometer; interference spectrometers utilizing a scanning Fourier Transform interferometer or stationary interferometer (e.g., Sagnac interferometer as described by Rafert, et al., Monolithic Fourier-Transform Imaging Spectrometer, Applied Optics, 34(31), pp 7228–30, 1995.); discrete light source spectrometers utilizing light emitting diodes, laser diodes or tunable diode lasers; and filter-based spectrometers utilizing acousto-optical transmission filters, liquid crystal filters, discrete optical filters, linear variable filters, or circular variable filters.
While each of these spectrometers is viable for generating spectra, each has shortcomings in terms of either optical throughput, stability, versatility, availability, size, or cost, depending upon the application. For example, in the case of using near-infrared spectra (1.25–2.5 μm) to measure analytes in biological media, Fourier Transform instruments that provide high optical throughput, high stability, high versatility and are readily available tend to be large and relatively expensive. While design improvements can be made to reduce size and cost while maintaining the other desirable characteristics of an FTIR system for this application, other methods for generating spectra with fewer, less costly parts could be competitive with the FTIR approach.
One such alternative is the use of optical filters. There are commercially available assemblies for spectral separation and detection that use optical filters, such as the MicroPac assembly available from Optical Coating Laboratories, Inc. (OCLI), as schematically shown in FIG. 1. The MicroPac assembly 10 receives light or radiation 12 through a dielectric linear variable filter (LVF) 14, micro-optics 16 and a detector array 18. The LVF 14 is a bandpass dielectric filter whose properties vary over its length such that the central wavelength of the pass band varies linearly across the filter 14. OCLI's MicroPac assembly 10 images the LVF 14 onto the detector array 18 to generate a spectrum of the light incident on the LVF 14. The cost of the MicroPac assembly 10 that can be used for visible and/or very near infrared regions is significantly affected by the price of the particular silicon detector array utilized. Due to the relative scarcity and high cost of NIR arrays that can detect light with a wavelength as long as 2.5 μm (e.g., InGaAs or PbS), the NIR embodiment of the OCLI MicroPac assembly is expensive.
During the proceedings of the MicroPac Conference hosted by OCLI in California, on May 11th and 12th, 2000, Shroder proposed the use of a fiber optic coupling to increase efficiencies of the assembly 10 by twenty (20) to forty (40) times. See Current Performance Results by Robert Shroder, MicroPac Forum Presentation, May 11, 2000. However, in all instances, the OCLI assembly 10 is used on the detector side of a spectrometer just prior to the detector array. It has not been suggested to use the LVF 14 on the illumination side of the system, prior to the sample under analysis, nor has it been suggested to include the LVF with an integrating chamber for coupling a light source to an encoded filter-based spectrometer.
Another approach used in spectrographic analysis is to incorporate a Hadamard or other encoding mechanism in a spectrometer to enable multiplexing and thereby increase the overall optical throughput of the system, as is known in the art. For example, Harwitt and Sloan (Harwit, M. and N. Sloan, Hadamard Transform Optics, pages 109–145, Academic Press, 1979) discuss the application of a Hadamard mask to either or both the entrance and exit planes of a grating spectrometer. However, such prior art does not indicate that the encoding can be combined favorably with a filter-based instrument.
U.S. Pat. No. 6,031,609, entitled “Fourier Transform Spectrometer using a Multielement Liquid Crystal Display,” teaches a system and method for combining prisms or gratings with a liquid crystal spatial light modulator in such a way as to create a Fourier Transform spectrometer. The advantages claimed for this system include increased signal-to-noise ratios over scanning dispersive instruments for a fixed integration time without any moving parts. However, such prior art does not indicate that multiple encoding methods and systems can be applied for encoding, or that the encoding can be combined favorably with a filter-based instrument.
From the foregoing, it should be apparent that there is an unmet need for an encoded filter-based spectrometer that optimizes optical throughput, stability, versatility, availability, size, and cost.